In-situ observation of eutectic growth during directional solidification of succinonitrile – (D)camphor- neopentyl glycol alloys under imposed velocity transients

In the present work are reported ground-reference results from a series of transient solidification experiments with the organic eutectic alloys SCN-23.6 DC (wt.%) and SCN-24.2DC-0.5NPG (wt.%), which have been performed in preparation of microgravity experiments using a new multi-user facility called Transparent Alloys or DIRSOL. In 1g lab conditions we investigated the response of rod-like eutectic patterns to imposed transients, involving defined acceleration and deceleration of the growth velocity. The patterns have been recorded in-situ in oblique view using a long distance optics and were analysed for a region of interest of 640 μm x 320 μm over long periods of time. The time dependent growth velocity and the associated acceleration / deceleration were determined using images from a side-view camera. The reported results mainly refer to the spacing distribution and the hexagonal order parameter S₀, dynamically attained at a defined time instant, i.e. at the moment when the local growth velocity reaches V = 40 nm s⁻¹ along deceleration 80 → 10 nm s⁻¹ or acceleration 10 → 80 nm s⁻¹ paths. The major result is that the hexagonal order is deteriorating under transient growth conditions compared to steady state conditions. We could not observe an increase of order for either imposed acceleration or deceleration, mainly because spacing adjustment is accomplished by abrupt morphological transitions which destroy the previous order. The abrupt morphological transitions are clearly identified as sharp events along an experiment cycle 80 → 10 → 80 nm s⁻¹ and could conveniently be seen in hysteresis loop diagrams. Upon deceleration the abrupt spacing adjustment event corresponds to a transition from fibrous to vermicular morphology. Upon acceleration the abrupt spacing adjustment event corresponds to spontaneous rod splitting.